Abstract:

A method for finding the correlation between tool PM (prevention
maintenance) and the product yield of the tool is disclosed. The method
uses a moving average method to magnify a curve trend that is formed by
the product yield data that is captured during a predetermined days
before PM and after PM. The magnified curve trend is shown by a
Cumulative sum chart. The Cumulative sum chart is analyzed for informing
related workers of the effect between the tool PM and the product yield,
so as to accurately estimate PM timing. Thereby, via the method, the
effect between the tool PM and the product yield may be determined, which
serves as an important reference for workers to execute further PM.

Claims:

1. A method for finding the correlation between prevention maintenance of
a tool and the product yield of the tool, the method comprising:(a)
collecting a plurality of product yield data of a plurality lots of
wafers that are correspondingly formed from being processed by a
plurality of tools of a production line;(b) according to the
aforementioned product yield data, respectively calculating a yield gap
between an average yield a predetermined days before the prevention
maintenance of the tool and an average yield a predetermined days after
the prevention maintenance of the tool;(c) selecting the tools that has
its product yield gap greater than or equal to a predetermined yield
threshold;(d) within the time frame between the predetermined days before
the prevention maintenance of the tool and the predetermined days after
the prevention maintenance of the tool, selecting the tools that has
processed wafer lots in numbers greater than or equal to a predetermined
lot count threshold;(e) finding all the product yield data generated at
different time for different wafer lots from each of the tools in step
(c) and step (d), and using a moving average method to magnify a curve
trend that is formed by the product yield data of each tool which has
been captured during the predetermined days before the prevention
maintenance of the tool and the product yield data of each tool which has
been captured during the predetermined days after the prevention
maintenance of the tool, so as to decrease the effect of data noise to
subsequent analysis;(f) displaying every one of the aforementioned
magnified curve trend via a format of cumulative sum chart
respectively;(g) through each of the cumulative sum chart, finding a
continuous downward trend period number from the product yield data
during the predetermined days before the prevention maintenance of the
tool, furthermore finding a continuous upward trend period number from
the product yield data during the predetermined days after the prevention
maintenance, so as to find the tools that has its sum of the continuous
downward trend period number and the continuous upward trend period
number greater than or equal to a predetermined period threshold; and(h)
for those tools that are greater than or equal to the aforementioned
predetermined period threshold, sending a notification information to
related workers regarding the effect between the prevention maintenance
for those tools and the product yield of those tools.

2. The method according to claim 1, wherein the predetermined days before
the prevention maintenance of the tool and the predetermined days after
the prevention maintenance of the tool are equal number of days.

3. The method according to claim 2, wherein the predetermined days before
the prevention maintenance of the tool and the predetermined days after
the prevention maintenance of the tool are both 3 days.

4. The method according to claim 1, wherein a selection equation used in
step (c) for selecting the tools
comprises:|Mean_yield(PMpre3)-Mean_yield(PMpost3)|≧yield
gap threshold,wherein Mean_yield(PMpre3) is the average yield a
predetermined days before the prevention maintenance, Mean_yield
(PMpost3) is the average yield a predetermined days after the
prevention maintenance, and the yield gap of the two average yields must
be greater than or equal to the aforementioned predetermined yield gap
threshold for a tool to be selected.

5. The method according to claim 4, wherein the predetermined days before
the prevention maintenance of the tool and the predetermined days after
the prevention maintenance of the tool are both 3 days.

6. The method according to claim 1, wherein a selection equation used in
step (d) for selecting the tools comprises:(Lot countpre3) and (Lot
countpost3)≧Lot count threshold,wherein (Lot countpre3)
and (Lot countpost3) are respectively processed wafer lot numbers
during the time frame between the predetermined days before the
prevention maintenance of the tool and the predetermined days after the
prevention maintenance of the tool, and the wafer lot numbers must be
greater than or equal to the aforementioned predetermined lot count
threshold for a tool to be selected.

7. The method according to claim 6, wherein the predetermined days before
the prevention maintenance of the tool and the predetermined days after
the prevention maintenance of the tool are both 3 days.

8. The method according to claim 1, wherein a magnification equation used
in step (e) for magnifying the curve trend comprises: i = 1 n -
threshold + 1 Mean ( YB period ( i ) + YB period (
i + 1 ) + + YB period ( i + threshold - 1 ) ) ,
##EQU00002## wherein the magnification equation magnifies the curve trend
that is formed by the product yield data of each tool which has been
captured during the predetermined days before the prevention maintenance
of the tool and the product yield data of each tool which has been
captured during the predetermined days after the prevention maintenance
of the tool, and YBperiod is the product yield of each of the lots
of wafers, n is the total number of lots for the wafers, and threshold is
the number of period used by the moving average method.

9. The method according to claim 8, wherein the predetermined days before
the prevention maintenance of the tool and the predetermined days after
the prevention maintenance of the tool are both 3 days.

10. The method according to claim 1, wherein a determination equation used
in step (g) for finding the tools comprises:(Trend down
periods.sub.prepPM+Trend up periods.sub.postPM)≧period
threshold,wherein Trend down periods.sub.prePM is the continuous downward
trend period number from the product yield data during the predetermined
days before the prevention maintenance of the tool, Trend up
periods.sub.postPM is the continuous upward trend period number from the
product yield data during the predetermined days after the prevention
maintenance, and the sum of the two period numbers must be greater than
or equal to the predetermined period threshold for finding a tool.

[0003]The present invention relates to a method for finding the
correlation between two subjects, in particular, to a method for finding
the correlation between tool prevention maintenance (PM) and the product
yield of the tool.

[0004]2. Description of Related Art

[0005]In terms of equipment manufacturing, the prevention maintenance (PM)
for a tool is very important. A timely prevention maintenance schedule
can effectively increase the productivity of the tool, the availability
of the tool, and the associated yield of the overall equipment
manufacturing. On the other hand, an improper prevention maintenance
schedule besides resulting in decreased productivity and decreased yield
for the associated tool, may also cause the tool itself to have a chance
of malfunctioning.

SUMMARY OF THE INVENTION

[0006]In view of the aforementioned issues, the present invention provides
a method for finding the correlation between a prevention maintenance
(PM) of a tool and the product yield of the tool, so that via the method
the effect between the tool PM and the product yield thereof may be
determined, which serves as an important reference for workers to execute
further PM.

[0007]To achieve the above-mentioned objectives, the present invention
provides a method for finding the correlation between a prevention
maintenance of a tool and the product yield of the tool, the method
includes the following steps: first (a) collecting a plurality of product
yield data of a plurality lots of wafers that are correspondingly formed
from being processed by a plurality of tools of a production line; next,
(b) according to the aforementioned product yield data, respectively
calculating a yield gap between an average yield a predetermined days
before the prevention maintenance of the tool and an average yield a
predetermined days after the prevention maintenance of the tool; then (c)
selecting the tools that has it product yield gap greater than or equal
to a predetermined yield threshold.

[0008]Then, (d) within the time frame between the predetermined days
before the prevention maintenance of the tool and the predetermine days
after the prevention maintenance of the tool, selecting the tools that
has processed wafer lots in numbers greater than or equal to a
predetermined lot count threshold; which is followed by (e), finding all
the product yield data generated at different time for different wafer
lots from each of the tools in step (c) and step (d) and using a moving
average method to magnify a curve trend that is formed by the product
yield data of each tool which has been captured during the predetermined
days before the prevention maintenance of the of the tool and the product
yield data of each tool which has been captured during the predetermined
days after the prevention maintenance of the tool, so as to decrease the
effect of data noise to subsequent analysis; then, (f) displaying every
one of the aforementioned magnified curve trend via a format of
cumulative sum chart respectively; (g) through each of the cumulative sum
chart, finding a continuous downward trend period number from the product
yield data during the predetermined days before the prevention
maintenance of the tool, furthermore finding a continuous upward trend
period number form the product yield data during the predetermined days
after the prevention maintenance, so as to find the tools that has its
sum of the continuous downward trend period number and the continuous
upward trend period number greater than or equal to a predetermined
period threshold; lastly, (h) for those tools that are greater than or
equal to the aforementioned predetermined period threshold, sending a
notification information to related workers regarding the effect between
the prevention maintenance for those tools and the product yield of those
tools.

[0009]Therefore, the efficacy of the present invention is such that:
through the feedback system of the present invention, related workers
shall be informed of the effect between the prevention maintenance of a
tool and the product yield of the tool, so as to more accurately manage
the timing for the prevention maintenance of the tool and thereby
resulting in greater product yield.

[0010]In order to further understand the techniques, means, and effects
the present invention takes for achieving the prescribed objectives, the
following detailed descriptions and appended drawings are hereby
referred; such that, through which the purposes, features, and aspects of
the present invention can be thoroughly and concretely appreciated;
however, the appended drawings are merely provided for reference and
illustration, without any intention to be used for limiting the present
invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIGS. 1A and 1B show a flowchart of a method for finding the
correlation between the prevention maintenance of a tool and the product
yield of the tool according to the present invention;

[0012]FIG. 2A shows a yield trend chart formed via the method of finding
the correlation between the prevention maintenance of a tool and the
product yield of the tool according to the present invention, wherein the
method did not utilize moving average method;

[0013]FIG. 2B shows a yield trend chart formed via the method of finding
the correlation between the prevention maintenance of a tool and the
product yield of the tool according to the present invention, wherein the
method has utilized moving average method;

[0014]FIG. 3A shows a cumulative sum (CUSUM) chart formed via the method
of finding the correlation between the prevention maintenance of a tool
and the product yield of the tool according to the present invention,
wherein the method did not utilize moving average method; and

[0015]FIG. 3B shows a cumulative sum (CUSUM) chart formed via the method
of finding the correlation between the prevention maintenance of a tool
and the product yield of the tool according to the present invention,
wherein the method has utilized moving average method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016]Next, the method for finding the correlation between a prevention
maintenance (PM) of a tool and the product yield of the tool in
accordance of the present invention is explained in conjunction with
FIGS. 1A, 1B, 2A, 2B, 3A, and 3B.

[0017]First, collecting a plurality of product yield data of a plurality
lots of wafers that are correspondingly formed from being processed by a
plurality of tools of a production line (Step S100). In other words, due
to consideration that a production line would generally have various
processing lines, therefore different lots of wafers may be assigned to
different tools on the production line, so that the product yield rate
for each lot of wafers would generally be different, for example the
difference of wear and tear on each tool may effect to result in product
yield rates with different values.

[0018]Next, according to the aforementioned product yield data,
respectively calculating a yield gap between an average yield a
predetermined days before the prevention maintenance of the tool and an
average yield a predetermined days after the prevention maintenance of
the tool (Step S102); where the predetermined days before the prevention
maintenance of the tool and the predetermined days after the prevention
maintenance of the tool are equal number of days. As one example for the
present invention, the predetermined days before the prevention
maintenance of the tool and the predetermine days after the prevention
maintenance of the tool are both 3 days.

[0019]Next, selecting the tools that has it s product yield gap greater
than or equal to a predetermined yield threshold (Step S104). As an
example, a selection equation used in step (c) for selecting the tools
may be as follow:

|Mean_yield(PMpre3)-Mean_yield(PMpost3)|≧yield gap
threshold

wherein Mean_yield(PMpre3) is the average yield a predetermined days
before the prevention maintenance, Mean_yield(PMpost3) is the
average yield a predetermined days after the prevention maintenance, and
the yield gap of the two average yields must be greater than or equal to
the aforementioned predetermined yield gap threshold for a tool to be
selected.

[0020]Then, within the time frame between the predetermined days before
the prevention maintenance of the tool and the predetermined days after
the prevention maintenance of the tool, selecting the tools that has
processed wafer lots in numbers greater than or equal to a predetermined
lot count threshold (Step S106). As an example, the above mentioned Step
S106 may use a selection equation such as the one described below for
selecting the tools:

(Lot countpre3) and (Lot countpost3)≧Lot count threshold,

wherein (Lot countpre3) and (Lot countpost3) are respectively
processed wafer lot numbers during the time frame between the
predetermined days before the prevention maintenance of the tool and the
predetermined days after the prevention maintenance of the tool, and the
wafer lot numbers must be greater than or equal to the aforementioned
predetermined lot count threshold for a tool to be selected.

[0021]Next, finding all the product yield data generated at different time
for different wafer lots from each of the tools in Step S104 and Step
S106, and using a moving average method to magnify a curve trend that is
formed by the product yield data of each tool which has been captured
during the predetermined days before the prevention maintenance of the
tool and the product yield data of each tool which has been captured
during the predetermined days after the prevention maintenance of the
tool, so as to decrease the effect of the data noise to subsequent
analysis (Step S108). As an example, a magnification equation used in
Step S108 may be as follows:

wherein the magnification equation magnifies the curve trend that is
formed by the product yield data of each tool which has been captured
during the predetermined days before the prevention maintenance of the
tool and the product yield data of each tool which has been captured
during the predetermined days after the is prevention maintenance of the
tool, and YBperiod is the product yield of each of the lots of
wafers, n is the total number of lots for the wafers, and threshold is
the number of period used by the moving average method.

[0022]As an example, if the moving average method has not been used for
"finding all the product yield data generated at different time for
different wafer lots from each of the tools in Step S104 and Step S106",
then the product yield data that has been found would be as shown in FIG.
2A, which cannot clearly and efficiently find the upward trend and
downward trend that occurs 3 days before and 3 days after the prevention
maintenance that took place at the time of 2008.9/25.13:22:39; thereby
the product yield data shown in FIG. 2A has no reference value. However,
on the other hand if the curve trend of the product yield data has been
magnified, then the resulting graph (as shown in FIG. 2B) representing
the product yield data can clearly and effectively show the upward trend
and downwards trend that occurs 3 days before and 3 days after the
prevention maintenance that took place at the time of 2008.9/25.13:22:39.

[0023]Then, displaying everyone of the aforementioned magnified curve
trend via a format of cumulative sum (CUSUM) chart respectively (Step
S110). As an example, if the moving average method was not used in Step
S108, then the CUSUM chart (as shown in FIG. 3A) still may not be used to
effectively read the upward trend and downward trend that occurs 3 days
before and 3 days after the prevention maintenance that took place at the
time of 2008.9/25.13:22:39. However, if the moving average method was
used in Step S108, then the CUSUM chart (as shown in FIG. 3B) may be used
to effectively read the upward trend and the downward trend that occurs 3
days before and 3 days after the prevention maintenance that took place
at the time of 2008.9/25.13:22:39.

[0024]Next, through each of the cumulative sum chart, finding a continuous
downward trend period number from the product yield data during the
predetermined days before the prevention maintenance of the tool,
furthermore finding a continuous upward trend period number from the
product yield data during the predetermined days after the prevention
maintenance, so as to find the tools that has its um of the continuous
downward trend period number and the continuous upward trend period
number greater than or equal to a predetermined period threshold (Step
S112). As an example, the above mentioned Step S112 may use a
determination equation such as the one describe below for finding the
tools:

(Trend down periods.sub.prePM+Trend up periods.sub.postPM)≧period
threshold,

[0025]wherein Trend down periods.sub.prePM is the continuous downward
trend period number from the product yield data during the predetermined
days before the prevention maintenance of the tool, Trend up
periods.sub.postPM is the continuous upward trend period number from the
product yield data during the predetermined days after the prevention
maintenance, and the sum of the two period numbers must be greater than
or equal to the predetermined period threshold for finding a tool.

[0026]Finally, for those tools that are greater than or equal to the
aforementioned predetermined period threshold, sending a notification
information to related workers regarding the effect between the
prevention maintenance for those tools and the product yield of those
tools (Step 112). In other words, through the feedback system of the
present invention, related workers shall be informed of the effect
between the prevention maintenance of a tool and the product yield of the
tool, so as to more accurately manage the timing for the prevention
maintenance of the tool and thereby resulting in greater product yield.

[0027]As described above, through using the moving average method, the
present invention magnifies the product yield data 3 days before and 3
days after the prevention maintenance respectively for each tool, then
subsequently display the magnification in terms of a CUSUM chart; through
analysis of each of the CUSUM chart, workers may be informed of the
effect between the prevention maintenance of the tool and the product
yield of the tool, so as to accurately estimate proper future prevention
maintenance timing. Thereby, via the method, the effect between the tool
prevention maintenance and the product yield may be determined, which
serves as an important reference for workers to execute further
prevention maintenance.

[0028]The above-mentioned descriptions represent merely the preferred
embodiment of the present invention, without any intention to limit the
scope of the present invention thereto. Various equivalent changes,
alternations, or modifications based on the claims of present invention
are all consequently viewed as being embraced by the scope of the present
invention.